There are intense activities in recent years for development of fuel cells that generate electric power from hydrogen and oxygen in the air. These activities are motivated by three reasons below.
First, they are excellent power generating methods in view of the environmental protection, because substance being released during generation of electricity is only water. Secondly, they save energy because they are fundamentally superior in efficiency of generating the electric energy. Thirdly, they have an advantage of using even thermal energy by collecting heat produced during generation of electricity. They therefore hold expectation as the last resort to solve the energy and environmental problems in the global scale.
Such fuel cell systems as discussed above are considered useful for application to home-use cogeneration systems as well as automobiles.
The fuel cell systems are expected to grow more and more in the future as alternative sources of energy to replace the existing thermal power generation and gasoline engines that use fossil fuel. Because the fuel cells use hydrogen for the fuel, it is important that they are provided with safety measures. In other words, it is indispensable for the fuel cells to be equipped with hydrogen concentration detectors for detecting leakage of hydrogen to ensure the safety. There have been hitherto proposed some devices as such hydrogen concentration detectors that detect hydrogen concentration by monitoring temperature change of heater elements based on the principle that a coefficient of thermal conductivity of hydrogen is extremely large as compared to other gases. When hydrogen gas reaches a heater element which is in a state of thermal equilibrium in the air, for instance, there occurs a change in amount of heat being taken away from the heater element, and causes it to lose the thermal equilibrium. As a result, temperature of the heater element changes according to a level of the hydrogen concentration. The hydrogen concentration detector uses a temperature sensor element to electrically detect this temperature change. Platinum temperature-measuring elements are well known as the heater elements, and the temperature sensor elements used for such hydrogen concentration detectors. Since platinum has a relatively high specific resistance among metals, it generates heat in itself when an electric current is fed through. In addition, because platinum also has a relatively high temperature coefficient of resistance among metals, it can be used for detection of temperature change corresponding to the hydrogen concentration through a change in resistance. A gas detector contrived to use the above method is disclosed in the official publication of unexamined Japanese utility model, No. S62-12861. It uses two detectors of different heating temperatures, and a factor multiplier and an arithmetic unit to solve a set of simultaneous equations obtained from outputs of the two detectors. This gas detector can thus measure a level of concentration of the gas to be detected while eliminating influence of interfering gases.
However, the above gas detector of the prior art shows a drawback when there is water vapor present as an interfering gas in the gas to be detected. In other words, a resistance of platinum changes precisely in proportion to the level of hydrogen concentration only if there is no water vapor. However, the water vapor, if present, also changes the resistance of platinum. As a result, the gas detector is unable to determine what has caused the change in resistance, whether it is due to hydrogen, water vapor, or both of them in coexistence. It is for this reason to solve the above problem that the gas detector of the prior art makes arithmetic operation of the simultaneous equations to eliminate influence of the interfering gases. However, water vapor shows such characteristics that its coefficient of thermal conductivity rises with increase in absolute humidity, but start descending once the coefficient value reaches a peak level in an atmosphere in which the water vapor having polarities is mixed with unpolarized air, hydrogen and the like, although the coefficient value of water vapor is extremely smaller than that of hydrogen when present separately.
Therefore, in the case where a large amount of water vapor is assumed present in relation to hydrogen, a coefficient of thermal conductivity in the mixture of the water vapor and hydrogen rises once with increase in absolute humidity and descends after it reaches the peak level, as previously stated, when detecting hydrogen leakage. That is, the mixture shows a characteristic that the coefficient of thermal conductivity changes in a form of quadratic curve. Therefore, if a gas detector is adapted to use the prior art technique of solving the simultaneous equations as described above, it calculates only a level of hydrogen through arithmetic operation of the simultaneous equations in two unknowns, and has the following problems. That is, the operation is very complex as a signal processing technique of the sensor, and it gives rise to problems with regard to detecting accuracy, multiplicity of uses, cost of putting it to practical use, and so forth.
Accordingly, it is an object of the present invention to provide a gas detector having a capability of detecting hydrogen concentration by distinguishing it from moisture in the environment wherein the hydrogen coexists with water vapor.